PEM fuel cell

A PEM fuel cell consists of a polymer electrolyte
membrane sandwiched between an anode (negatively charged electrode)
and a cathode (positively charged electrode). The processes that take
place in the fuel cell are as follows: 1. Hydrogen fuel is channeled
through field flow plates to the anode on one side of the fuel cell,
while oxygen from the air is channeled to the cathode on the other
side of the cell. 2. At the anode, a platinum catalyst causes the
hydrogen to split into positive hydrogen ions (protons) and negatively
charged electrons. 3. The Polymer Electrolyte Membrane (PEM) allows
only the positively charged ions to pass through it to the cathode.
The negatively charged electrons must travel along an external circuit
to the cathode, creating an electrical current. 4. At the cathode,
the electrons and positively charged hydrogen ions combine with oxygen
to form water, which flows out of the cell.

Polymer electrolyte membrane (PEM) fuel cells – also called proton
exchange membrane fuel cells – deliver high-power density and offer
the advantages of low weight and volume, compared with other fuel
cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing
a platinum catalyst. They need only hydrogen, oxygen from the air, and water
to operate and do not require corrosive fluids like some fuel cells. They
are typically fueled with pure hydrogen supplied from storage tanks or on-board reformers.

Polymer electrolyte membrane fuel cells operate at relatively low temperatures,
around 80°C (176°F). Low-temperature operation allows them to start
quickly (less warm-up time) and results in less wear on system components,
resulting in better durability. However, it requires that a noble-metal
catalyst (typically platinum) be used to separate the hydrogen's electrons
and protons, adding to system cost. The platinum catalyst is also extremely
sensitive to carbon monoxide (CO)
poisoning, making it necessary to employ an additional reactor to reduce
CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds
cost. Developers are currently exploring platinum/ruthenium catalysts that
are more resistant to CO.

PEM fuel cells are used primarily for transportation applications and some
stationary applications. Due to their fast startup time, low sensitivity
to orientation, and favorable power-to-weight ratio, PEM fuel cells are
particularly suitable for use in passenger vehicles, such as cars and buses.

A significant barrier to using these fuel cells in vehicles is hydrogen
storage. Most fuel cell vehicles (FCVs) powered by pure hydrogen must store
the hydrogen on-board as a compressed gas in pressurized tanks. Due to the
low-energy density of hydrogen, it is difficult to store enough hydrogen
on-board to allow vehicles to travel the same distance as gasoline-powered
vehicles before refueling, typically 300–400 miles. Higher-density
liquid fuels, such as methanol, ethanol, natural gas, liquefied petroleum
gas, and gasoline, can be used for fuel, but the vehicles must have an on-board
fuel processor to reform the methanol to hydrogen. This requirement increases
costs and maintenance. The reformer also releases carbon dioxide (a greenhouse
gas), though less than that emitted from current gasoline-powered engines.